Reactions of haloarenes

Aryl halides are far less reactive than alkyl halides towards nucleophilic substitution, for four reasons listed below.

Resonance: the halogen's lone pairs conjugate with the ring's π electrons, giving the C–X bond partial double bond character that is harder to break.

Hybridisation: the sp² carbon has more s-character, is more electronegative and holds the bonding pair tighter than an sp³ carbon. C–Cl is 169 pm in a haloarene against 177 pm in a haloalkane, and the shorter bond is harder to break.

A phenyl cation from self-ionisation gets no resonance stabilisation, which rules out SN1; and the electron-rich nucleophile is repelled by the electron-rich ring.

Chlorobenzene becomes phenol only under harsh conditions: aqueous NaOH at 623 K and 300 atmospheres.

An –NO₂ group ortho or para to the halogen raises reactivity by withdrawing electron density and stabilising, through resonance, the negative charge in the carbanion intermediate. A meta –NO₂ has no effect, because none of the resonance structures then puts the negative charge on the carbon carrying –NO₂.

Haloarenes undergo the usual electrophilic substitutions of benzene: halogenation, nitration, sulphonation and Friedel-Crafts reactions. The halogen is slightly deactivating yet ortho, para-directing.

Why both: the –I effect withdraws electrons and deactivates the ring, so reactions are slower and need harsher conditions than with benzene. Resonance releases electrons to the ortho and para positions and stabilises attack there. The stronger inductive effect sets reactivity; resonance sets orientation.

Wurtz-Fittig reaction: an alkyl halide and an aryl halide with sodium in dry ether give an alkylarene. Fittig reaction: aryl halides alone with sodium in dry ether join two aryl groups (e.g. biphenyl).